1. Field of the Invention
The present invention relates to an optical pickup used in an optical disk system, such as a compact disk, a video disk, etc. More particularly, the present invention relates to an optical pickup comprising a hologram element-incorporating semiconductor laser device.
2. Description of the Related Art
An optical pickup comprising a semiconductor laser device is used to read out information stored in an optical disk, such as a compact disk, etc. In the optical pickup, light emitted from the semiconductor laser device is split by a diffraction grating of a hologram element into one main beam and two subbeams (tracking beams) which are brought onto an optical disk. The main beam and the subbeams are reflected on the optical disk, and each reflected beam is further split by a hologram of the hologram element into two beams, which are brought to a light receiving element or a signal processing integrated circuit with a light receiving element. Thereafter, based on an output signal from the signal processing integrated circuit, a tracking information signal, etc. used for accurately reading out signals recorded in the optical disk can be obtained.
FIG. 4 is a schematic diagram showing the optical system of a conventional three-beam hologram optical pickup.
This optical pickup has a semiconductor laser chip (LD) 6. Light emitted from the semiconductor laser chip 6 is split by a tracking beam generating diffraction grating 5, provided on the rear side of a hologram element (not shown), into three beams, i.e., two subbeams for tracking and one main beam for reading information signals. This light passes through a hologram 4 provided on the hologram element as zero-order light, and is then converted by a collimator lens 3 to parallel light. The parallel light is condensed by an objective lens 2 onto a disk 1. The light condensed onto the disk 1 is modulated by pits on the disk 1 and reflected from the disk 1. The reflected light from the disk 1 passes through the objective lens 2 and the collimator lens 3 in this order, and is then diffracted by the hologram 4 and introduced into a five-way split photodiode 7 as first-order diffracted light.
This five-way split photodiode 7 has five optical detectors D1 to D5. The five-way split photodiode 7 has a rectangular region which is illuminated by light. The region is divided into three equal parts which are strip regions extending in a longitudinal direction. Two opposite regions are first and fifth optical detectors D1 and D5. A middle strip region is further divided into two equal parts in a transverse direction. One of the two regions is a fourth optical detector D4. The other region is further divided into two parts in a longitudinal direction, which are second and third optical detectors D2 and D3.
The hologram 4 has two regions 4a and 4b which have different grating pitches. The main beam of reflected light entering the region 4a is condensed onto the splitting line between the second optical detector D2 and the third optical detector D3 of the five-way split photodiode 7. The main beam of reflected light entering the region 4b is condensed onto the fourth optical detector D4. Further, the two subbeams of reflected light entering the region 4a are condensed onto the opposite first and fifth optical detectors D1 and D5, so that two beam spots are formed on each of the optical detectors D1 and D5.
As described above, the beam spots of reflected light condensed on the optical detectors D1 to D5 of the five-way split photodiode 7 vary depending on the focusing conditions of the light brought onto the disk 1 as shown in FIGS. 5A to 5C. FIG. 5A shows spots when light is focused beyond the optical disk 1. FIG. 5B shows spots when light is properly focused on the optical disk 1. FIG. 5C shows spots when light is focused before the disk 1.
The outputs of the optical detectors D1 to D5 of the five-way split photodiodes 7 are represented by S1, S2, S3, S4 and S5, respectively. A focus error signal FES is given by the difference between the outputs of the second optical detector D2 and the third optical detector D3:FES=S2−S3
A tracking error is detected by a so-called three-beam method. The tracking subbeams are condensed onto the optical detectors D1 and D5. A tracking error signal TES is given by the difference between the outputs of the optical detectors D1 and D5:TES=S1−S5
A reproduction signal RF is given by the sum of the outputs of the second, third and fourth optical detectors D2,D3 and D4:RF=S2+S3+S4
In a hologram optical pickup using the conventional three-beam method, the hologram 4 includes two regions 4a and 4b having different grating pitches. Light beams which pass through the regions 4a and 4b of the hologram 4 after reflection on the optical disk 1 have different diffraction angles. Therefore, the light beams which have passed through the regions 4a and 4b are diffracted at a smaller angle and a larger angle in one direction with respect to the hologram 4.
The grating of the hologram 4 is typically formed of grooves which are formed by patterning using a photoetching technique. When the two regions 4a and 4b having different grating pitches are formed by patterning, the depth of the grooves and the angle of grating vary in each of the regions 4a and 4b, depending on an etching rate, etc.
If the groove depth and grating angle vary in each of the regions 4a and 4b, the variations appear as the difference in the intensity of diffracted light between the main beam and the subbeams, i.e., the difference in diffraction efficiency. As a result, the optical intensities of reflected light beams entering the optical detectors D1 to D5 are unbalanced, so that offset develops in the tracking error signal TES. In this case, characteristics of the hologram optical pickup are likely to be degraded.